In recent years, the use of multi-CPU (Central Processing Unit) systems (also referred to as multi-processor systems) has been popularized. For example, a NUMA (Non-Uniform Memory Access) computer system having two or more nodes coupled by a node interconnect switch that interconnects nodes has been disclosed.
Each node in the above NUMA computer system may be equal. Each node is coupled between a processing unit coupled to a local interconnect and the node interconnect switch. A node controller functions as a local agent for other nodes by transmitting a selecting instruction received on the local interconnect to the other nodes via the node interconnect switch and by transmitting the received selecting instruction onto the local interconnect.
In addition, in a number of computer systems in recent year, the CPU includes cache memory. Various technologies to maintain cache consistency (also known as cache coherency), which is the consistency of data among cache memories, when multiple devices share a main memory, have been known.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-44455    Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-342162    Patent Document 3: Japanese Laid-Open Patent Publication No. 09-237223